MIT has pulled out all the stops in an energy scavenging circuit for piezoelectric power sources
The traditional way to extract power from these is simply to connect a bridge rectifier to the piezo generator, followed by a reservoir capacitor.
However, a piezo generator is effectively an AC current source in parallel with a capacitor, so on every half cycle the current source wastes energy discharging the internal capacitance, then charging it to the opposite polarity.
Described at ISSCC (International Solid-State Circuits Conference) in San Francisco this week, MIT retains the rectifier and reservoir, but switches an inductor across the piezoelectric generator output.